This invention relates to a time-division multiplexer and, more particularly, to a time-division multiplexer having a time-division bus structure equipped with two independent buses which transport time-division multiplexed data, and a time-slot assignment unit which freely interchanges time slots between buses, wherein the buses are capable of being connected to each other via the time-slot assignment unit.
The present invention relates to a time division multiplexer based upon a 64 Kbps PCM structure and is applicable under the V5.2 protocol. A number of international and national standards bodies provide recommendations and standards for a common interface and coding between time-division multiplexers. One such body is the International Telecommunication Union, which publishes a large number of recommendations under the ITU-T standardization sector. An ITU-T publication G.797 contains many recommendations relating to types of services, interfaces and performance aspects that the time-division multiplexer should support. However, ITU-T G.797 does not define the means for implementing the equipment and leaves this to the manufacturer.
The present invention concerns the implementation of time-division multiplexers. A favorable application of time-division multiplexers is to extend the range of digital switching systems by using remote time-division multiplexers connected by fiber-optic cables. To minimize the overall capacity in the fiber-optic cable and maximize the number of services offered, a level of concentration is employed. Concentration is on a first-come first-served basis. When trunk capacity is reached, the next service requesting a circuit switched connection will be refused. A particular signaling standard being adopted by digital switching systems and time-division multiplexers is the ETSI V5.2, which is a common channel signaling standard.
A time-division multiplexer is composed of a group of units mounted at right angles on a back wiring board (BWB). As shown in FIG. 18, the time-division multiplexer has a plurality of tributary units TRB1-TRBn, at least one trunk unit TRU and a time-slot assignment unit TSA for assigning time slots.
The trunk unit TRU operates as an optical interface when time-division data or a signaling message is exchanged with another time-division multiplexer via an optical fiber OFB. The tributary units TRB1-TRBn operate as service interfaces for connecting subscribers or customer terminals, convert the format of time-division multiplexed data in such a manner that the data will suit the customer interface, and send/receive data and signaling messages to and from lines L1 to LN. The time-slot assignment unit TSA time-division multiplexes N-channel data and demultiplexes data that has been time-division multiplexed. Connected to the tributary units TRB1-TRBn are, by way of example, a 64 Kbps telephone line, a 2B+D (144 Kbps) ISDN basic rate line, an ISDN primary rate line, a 64 Kbps analog leased line and a X2.1 line of Nxc2x764 Kbps, etc.
FIG. 19 is a block diagram of a network which employs time-division multiplexers as digital subscriber line (DSL) equipment. A time-division multiplexer DSL-R on the subscriber side accommodates subscribers on N channels, time-division multiplexes the data on the N channels and sends the multiplexed data to a time-division multiplexer DSL-C on the trunk side via a digital transmission line (optical fiber) OFB. The time-division multiplexer DSL-C on the trunk side demultiplexes the time-division multiplexed data into N channels and inputs the data to an exchange EXC. Further, the time-division multiplexer DSL-C on the trunk side time-division multiplexes N-channel data from the exchange EXC and sends the multiplexed data to the time-division multiplexer DSL-R on the subscriber side. The latter then demultiplexes this time-division multiplexed data to N channels and sends the data to the subscribers.
FIG. 20 illustrates the manner in which a time-division multiplexer is utilized as an access network unit for accessing an SDH network. Here SDHN represents a ring-shaped SDH network, AN an access network unit, FLX an SDH interface unit and LE a local exchange. The access network unit AN time-division multiplexes N-channel data, sends this time-division multiplexed data to the SDH network SDHN via optical fiber OFB, demultiplexes multiplexed data from the SDH network SDHN and sends the demultiplexed data to the subscriber side.
FIG. 21 is a diagram showing the structure of a frame applied under ITU-T G.704. Here one frame is composed of 32 time slots and one multiframe is composed of 16 frames. The duration of one frame is 125 xcexcs and the duration of one multiframe is 2 ms. The starting time slot of each of even-numbered frames (F0, F2, . . . , F14) is used as a sync time slot, and the starting time slot of each of odd-numbered frame (F1, F3, . . . , F15) is used for transmission of a predetermined signal (alarm, a particular pattern for CRC, etc.). Further, the 16th time slot of the 0th frame (F0) is used for multiframe alignment, and the 16th time slot of each of the remaining frames (F1, F2, . . . , F15) is used for communication of signaling messages. Since two channel""s worth of signaling messages are transmitted by a single 16th time slot (eight bits), 30 channel""s worth of signaling messages can be transmitted by a single multiframe. In other words, a signaling message is sent/received every 2 ms. A signaling message is for communicating the state of a telephone, such as on-hook, off-hook, ringing, etc.
The conventional time-division multiplexer assigns predetermined time slots to specific subscribers in a static manner. This means that if the maximum number of multiplexed channels of a digital transmission line is N, then subscribers on more than N channels cannot be accommodated. If an attempt is made to accommodate a larger number of subscribers with a low level of concentration, a problem that arises is an increase in the size of the equipment. Another problem with the conventional time-division multiplexer is difficulty in upgrading. For these reasons, there is demand for a time-division multiplexer that is capable of handling subscribers beyond the capacity of the transmission line and that can be upgraded with ease.
A further problem with the conventional time-division multiplexer is that each unit can be inserted only at a predetermined position on the back wiring board. This can lead to insertion at the wrong position and makes the insertion operation a troublesome one.
Accordingly, an object of the present invention is to provide a time-division multiplexer that is capable of handling subscribers beyond the capacity of a transmission line and that can be upgraded with ease.
Another object of the present invention is to provide a time-division multiplexer in which each unit can be inserted at any position on a back wiring board.
A further object of the present invention is to provide a time-division multiplexer having a function for giving notification of the congested state.
According to the present invention, the foregoing objects are attained by providing a time-division multiplexer having a trunk bus to which a trunk unit is connected and a tributary bus to which tributary units are connected, and a time-slot interchange unit provided between these buses, wherein interchange of time slots on the trunk side and time slots on the tributary side is performed dynamically by the time-slot interchange unit. Further, a common channel signaling controller is provided between the bus lines, wherein the common channel signaling controller performs control for dynamic assignment of time slots and sends a time-slot exchange request to the time-slot interchange unit.
By way of example, in the receive direction, the common channel signaling controller (1) obtains a vacant time slot of a receive tributary bus and adopts this time slot as a time slot TB on the tributary side; (2) sends a request for interchange of a time slot TA on the trunk side and the time slot TB on the tributary side to the time-slot interchange unit via a control bus; and (3) notifies a tributary unit of the time slot TB on the tributary side via the control bus; the time-slot interchange unit (4) interchanges the data in the time slot TA on the trunk side with that in the time slot TB on the tributary side based upon the time-slot interchange request; and the tributary unit (5) accepts data from the receive tributary bus at the timing of the notified time slot TB.
Further, in the transmit direction, the common channel signaling line (1) obtains a vacant time slot of a transmit tributary bus and adopts this time slot as a time slot TAxe2x80x2 on the tributary side; (2) sends a request for interchange of the time slot TAxe2x80x2 on the tributary side and a time slot TBxe2x80x2 on the trunk side to the time-slot interchange unit; and (3) notifies a tributary unit of the time slot TAxe2x80x2 on the tributary side; the time-slot interchange unit (4) interchanges the data in the time slot TAxe2x80x2 on the tributary side with that in the time slot TBxe2x80x2 on the trunk side based upon the time-slot interchange request; and the tributary unit (5) sends data to the transmit tributary bus at the timing of the notified time slot TAxe2x80x2.
Further, the common channel signaling controller (1) assigns a vacant time slot TD of a receive tributary bus to a call generated at the time of congestion; (2) sends a time-slot interchange unit a dynamic interchange request for interchanging a congestion-message send time slot TC and the time slot TD; and (3) notifies a tributary unit of the time slot TD on the tributary side; the time-slot interchange unit (4) interchanges a congestion message in a time slot TC on the trunk side with the time slot TD on the tributary side based upon the interchange request; and the tributary unit (5) accepts the congestion message from the receive tributary bus at the timing of a designated time slot TD.
If the arrangement described above is adopted, it is unnecessary to assign time slots to subscribers in a static manner. Communication can be achieved while assigning vacant time slots to subscribers dynamically, thereby making it possible to accommodate subscribers beyond the capacity of a transmission line.
Moreover, control for interchanging time slots can be performed in a short period of time and upgrading is simplified.
Further, since time slots are assigned dynamically, units such as trunk units and tributary units can be inserted at any positions on a back wiring board. In addition, the congested state of a time-division multiplexer can be reported to the subscriber, thereby making it possible to improve service.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.